Method for manufacturing superconducting wire

ABSTRACT

A compound superconducting wire comprising a matrix of CuX alloy and a multiplicity of Z 3  X filaments embedded in the matrix in a spaced relationship so as not to come into contact with each other wherein X is Sn or Ga and Z 3  X is Nb 3  Sn or V 3  Ga. In a precursor, therefore, a multiplicity of filaments of a base metal material Z such as Nb are arranged in a Cu base metal matrix concentrically in layers around a center core of a base metal material X such as Sn, in which the spacing between any adjacent filaments arranged in a former boundary region of an ε-phase bronze layer having a certain radius from the center produced when the precursor is preheat-treated at a temperature of 300° to 600° C. is made larger than the spacing between any adjacent filaments arranged in the other matrix regions. Also disclosed is a precursor wire wherein a composite of a Cu base metal material and a base metal material X capable of forming an alloy with the Cu base metal material is used as a matrix without arranging a core of the base metal material X at the center, and a multiplicity of filaments of a base metal material Z are embedded in the composite in a spaced relationship.

This application is a divisional of application Ser. No. 08/167,061,filed Dec. 16, 1993, now U.S. Pat. No. 5,753,862.

BACKGROUND OF THE INVENTION

The present invention relates to a compound superconducting wiresuitable for use in superconducting magnets of high magnetic field and amethod for manufacturing the same. Particularly, the present inventionis advantageously applicable to an Nb-Sn compound superconducting wireand a manufacturing method therefor.

The term "precursor of superconducting wire" or "superconducting wireprecursor" as used herein means a wire prior to undergoing a heattreatment, i.e., a wire prior to being imparted with asuperconductivity. The heat treated precursor to impart asuperconductivity, namely the wire converted into superconductor by theheat treatment, is termed a "superconducting wire".

Hitherto, superconducting wires have been manufactured by a methodso-called "internal diffusion method".

An Nb₃ Sn superconducting wire prepared by the internal tin diffusionmethod is known, for instance, from Japanese Patent Publication KokokuNo. 16141/1986. FIGS. 23 and 24 are explanatory sectional viewsrespectively illustrating an Nb₃ Sn superconducting wire precursor priorto undergoing a heat treatment according to a conventional internaldiffusion method described in Japanese Patent Publication Kokoku No.16141/1986 and an Nb₃ Sn superconducting wire after undergoing the heattreatment. In FIG. 23, denoted at numeral 41 is the superconducting wireprecursor prior to the heat treatment, at numeral 43 Nb base metalfilaments to be converted into a superconductor by the heat treatment,at numeral 44 barrier layer such as made of Ta, at numeral 45stabilizing layer such as made of oxygen free copper, at numeral 46 Cubase metal material, and at numeral 47 Sn base metal material. In FIG.24, denoted at numeral 48 is the superconducting wire after the heattreatment, at numeral 49 Nb₃ Sn filaments having a superconductivity,and at numeral 50 low-Sn-concentration bronze.

The superconducting wire 48 is obtained by subjecting thesuperconducting wire precursor 41 to a heat treatment at a hightemperature (typically ranging from 600° to 800° C.) to produce an Nb₃Sn compound in the Nb filaments 43.

The conventional method for manufacturing an Nb₃ Sn superconducting wireusing the internal diffusion method is as follows. First, an Nb basemetal material is inserted into a Cu tube and processed to decrease thearea in section to a certain diameter thereby giving a single core wire.This single core wire is cut into pieces having an appropriate lengthand a plurality of these wire pieces are stuffed into a container madeof Cu. In the center portion of this container is disposed a Cu basemetal material such as a Cu rod or a bundle of Cu wires. Air in thecontainer is evacuated, a cover is welded to the container to seal itup, and the thus treated container is extruded. Thereafter the Cu basemetal material in the center portion of the container is mechanicallyformed with an aperture. An Sn base metal material is inserted into thisaperture, and the Cu container is circumferentially covered with a tubemade of Ta or Nb, which is further covered with a Cu tube. The resultantis processed to reduce the cross-sectional area, typically is drawn to asmall size. When it is desired to produce a superconducting wire havinga heavy current capacity, a plurality of the thus obtained compositewires may be inserted into a Cu tube and then drawn to reduce thesection area. After drawing the wire to a final diameter, it is twistedand subjected to a heat treatment. This heat treatment causes Sn todiffuse into Cu existing therearound to form Cu-Sn alloy and further toreact with the Nb base metal filament to produce Nb₃ Sn either partiallyor entirely.

The superconducting wire precursor in the aforesaid internal diffusionmethod has a structure in which the Nb base metal filaments and a coreof the Sn base metal material are embedded in the Cu base metalmaterial. In particular, in order to increase as large as possible thecritical current density (Jc) which is one of the characteristics ofsuperconductivity, the Nb base metal filaments are embedded in the Cubase metal material as tightly as possible. The superconducting wirewhen cooled to the temperature of liquid helium is capable of allowingheavy current to flow therein without producing any electricalresistance.

As described above, in the compound superconducting wire manufactured bythe prior art internal diffusion method the Sn base metal material isdisposed at the center of the module and, hence, the space betweenadjacent Nb₃ Sn filaments is as narrow as about a half of the spacingbetween such filaments arranged in accordance with a usual bronzemethod. For this reason the Nb base metal filaments tend to come intocontact with each other to combine to each other when thesuperconducting wire precursor is heat-treated, thus resulting inincrease of the effective filament diameter (d_(eff)), which greatlyinfluences the electrical characteristics of the superconducting wire.The effective filament diameter is a value given by d_(eff) =3πΔM/4μ_(o) Jc where the sample is in the columnar form, ΔM represents thewidth of magnetization of the superconducting wire, and Jc representsthe critical current density in these conditions. As a result, a problemarises that although the resulting superconducting wire suffers noproblem with respect to DC current, it suffers a large hysteresis losswhen pulse current is made to flow therein with the result that thesuperconducting coil generates heat to degrade the stability thereof.

Further, since the Sn base metal material is centrally disposed in theinternal diffusion method, preheating for Sn diffusion produces agradient in Sn concentration. Accordingly, the composition of the Nb₃ Snfilements varies depending on the Sn concentration. This poses anotherproblem that the n value, which is one of the characteristics ofsuperconductivity, is undesirably decreased. The n value is an indexrepresenting the longitudinal homogeneity of a superconducting wire andappears in the formula, V∝I^(n). The superconducting characteristics ofthe wire become more excellent with increasing n value.

It is, therefore, a primary object of the present invention to provide acompound superconducting wire wherein the effective filament diameterthereof is decreased to a large extent with minimizing the decrease inthe critical current density Jc thereof, and the n value is increased.

A further object of the present invention is to provide a process forproducing a superconducting wire, especially an Nb-Sn compoundsuperconducting wire, having a high critical current density Jc, animproved effective filament diameter and an improved uniformity incomposition of superconductive compounds in the longitudinal directionof wire.

A still further object of the present invention is to provide animproved internal diffusion process for producing a superconducting wirewhich enables to prevent filaments of a metal to be converted into asuperconductive compound from contacting each other during the heattreatment without decreasing the number of the filaments arranged in amatrix metal.

Another object of the present invention is to provide a precursor ofcompound superconducting wire which can provide a superconducting wirehaving an improved critical current density Jc, a decreased hysteresisloss and an improved uniformity in composition of superconductivecompound by a heat treatment in a shortened period of time.

These and other objects of the present invention will become apparentfrom the description hereinafter.

SUMMARY OF THE INVENTION

In a superconducting wire precursor when heat-treated, an Sn base metalmaterial disposed at the center portion of the precursor wire isdiffused into a Cu base metal material during the heat treatment andreacts therewith to produce a bronze, and an ε-phase bronze is formedwithin a certain radius from the center in a certain temperature range.

The present inventors have found that the phenomenon that Nb₃ Snfilaments of a superconducting wire come into mutual contact, asencountered in heat-treating the precursor wire, appears particularly inan ε-phase boundary (outer periphery of ε-phase bronze layer) region,namely a region having a certain width in the radial direction in whichthe boundary of the ε-phase bronze locates, and the above objects can beachieved by preventing the Nb₃ Sn filaments from contacting each otherin this region.

In accordance with a first aspect of the present invention, there isprovided an Nb-Sn compound superconducting wire comprising a matrix of abronze having a low Sn concentration and a multiplicity of Nb₃ Snfilaments arranged in said matrix, wherein a center portion of thesuperconducting wire is formed of only said bronze, and said Nb₃ Snfilaments are concentrically outwardly arranged in layers around saidcenter portion separately from each other, and wherein the spacingbetween any adjacent Nb₃ Sn filaments disposed in a boundary region ofan ε-phase bronze layer produced when pre-heated at a temperature of300° to 600° C. is larger than that between any adjacent Nb₃ Snfilaments disposed in the other matrix portions.

The aforesaid Nb-Sn compound superconducting wire is prepared by aprocess which comprises the steps of:

(a) forming a composite body comprising a columnar Cu base metalmaterial, and a multiplicity of Nb base metal filaments embeddedseparately from each other in the Cu base metal material andconcentrically arranged in layers around a center portion thereof insuch a relationship that the spacing between any adjacent Nb base metalfilaments existing in a boundary region of an ε-phase bronze layerproduced when preheated at a temperature of from 300° to 600° C. islarger than that between any adjacent Nb base metal filaments existingin the other portions of the Cu base metal material;

(b) forming a through-hole in said center portion, typically in such amanner as subjecting the composite body to extrusion processing and thendrilling the center portion of the extrudate, or in such a manner asextruding the composite body into a tubular shape; and then inserting anSn base metal rod into the through-hole;

(c) drawing the resultant composite body to form a superconducting wireprecursor; and

(d) heat-treating the precursor.

The heat-treatment of the precursor makes Sn to diffuse from the Sn rodarranged in the center portion of the precursor into Cu to produce amatrix of a Cu-Sn bronze having a low concentration of Sn and to convertthe Nb filaments into Nb₃ Sn filaments. In the thus obtained Nb-Sncompound superconducting wire according to the present invention, thecenter portion corresponding to the Sn rod-inserted portion of theprecursor is made of only the low Sn concentration bronze, in otherwords, no Nb₃ Sn filament is present at the center portion. The bronzematrix comprises an outer low Sn concentration bronze region, an innerlow Sn concentration bronze region, and an annular ε-phase bronzeboundary region which is located between the outer and inner regions andin which the boundary of the ε-phase bronze produced at a temperature of300° to 600° C. and converted into α-phase bronze at a highertemperature appears and the Nb₃ Sn superconductor filaments are arrangedwith a larger spacing as compared to the arrangement in the other matrixregions.

According to the above-mentioned process, a superconducting wire isobtained in the state that the Nb₃ Sn filaments are not in contact witheach other. Accordingly, the superconducting filaments are preventedfrom mutually coupling while minimizing a reduction in Jc value, so thatthe effective filament diameter value is reduced. As a result, there isachieved a marked reduction in hysteresis loss upon conduction of pulsecurrent, leading to improvement in stability of a superconducting coil.

The mutual contact of the Nb₃ Sn filaments which is considered owing toexpansion, movement or swaying of filaments during the heat treatment ofthe precursor wire can also be decreased by not arranging the filamentsin the inside of the boundary of the ε-phase bronze layer. Accordingly,even if the spacing between the any adjacent filaments arranged in theboundary region of the ε-phase bronze layer is not increased, it ispossible to provide an improved Nb₃ Sn superconducting wire by arrangingthe Nb₃ Sn filaments in the same manner as in a conventional internaldiffusion method except that the filaments are not arranged in theinside of the boundary of the ε-phase bronze layer.

It has also been found that the Sn diffusion in the heat treatment stepcan be efficiently achieved to form a Cu-Sn alloy as a matrix and toconvert Nb filaments into Nb₃ Sn filaments by using a Cu-Sn compositematerial as a matrix for Nb filaments instead of arranging an Sn rod inthe center portion of the precursor. Thus, the spacing between adjacentNb filaments can be increased without decreasing the number of Nbfilaments to be arranged, and the Nb filaments can be prevented fromcontacting each other in the heat treatment of the precursor.

Thus, the present invention further relates to a superconducting wire,its precursor and a method for manufacturing the superconducting wire,wherein the problems associated with the ε-phase boundary are solved bydisposing composite bodies of a Cu base metal material and an Sn basemetal material in the overall module without disposing the Sn base metalmaterial in the center portion of the module.

This technique is also advantageously applicable to the preparation ofother compound type superconducting wires, e.g., V₃ Ga, Nb₃ Al and V₃Si.

Accordingly, in accordance with the second aspect of the presentinvention, there is provided a compound superconducting wire precursorcomprising a composite composed of a Cu base metal material and a firstbase metal material X capable of forming an alloy with the Cu base metalmaterial, and a multiplicity of filaments of a second base metalmaterial Z which are provided within the composite so as not to comeinto mutual contact.

The compound superconducting wire according to the second aspect of thepresent invention is obtained by heat-treating the aforesaidsuperconducting wire precursor, and thereby converting the filaments ofthe second base metal material Z into Z₃ X filaments and forming a Cu-Xalloy as the matrix for the Z₃ X filaments.

According to the second aspect of the present invention, thesuperconducting wire is prepared by a process comprising the steps of:

(A) forming a composite rod in which a mulitplicity of rods of a secondbase metal material Z are embedded in a composite body composed of a Cubase metal material and a first base metal material X capable of formingan alloy in combination with the Cu base metal material;

(B) drawing the composite rod to form a superconducting wire precursor;and

(C) heat-treating the superconducting wire precursor.

In the compound superconducting wire precursor according to the secondaspect of the present invention, since the first base metal material Xsuch as Sn or Ga is dispersedly arranged, there is no need to provide afirst base metal core such as Sn or Ga in the central portion of theprecursor wire as conducted in a known internal diffusion process and,hence, the area capable of being provided with filaments is increased bythe area of the central portion. This makes it possible to enlarge thespacing between adjacent filaments by about 30% as compared with thataccording to a conventional internal diffusion method. Consequentlythere is achieved a remarkable reduction in the probability of mutualcontact between superconducting filaments in the superconducting wireobtained by the heat treatment of the precursor, resulting in asubstantial reduction in the effective filament diameter value. Thisleads to a substantial reduction in hysteresis loss caused uponconduction of pulse current and, hence, contributes to realization of asuperconducting coil having an improved stability.

In addition, the distance of diffusion of the first base metal X such asSn or Ga is shortened to assure a uniform content of Sn or Ga after thediffusion. As a result, the composition of the Nb₃ Sn filaments or thelike to be produced is made homogeneous thereby improving the n valuewhich is one of the characteristics of superconductivity. Further thepreheating time required for diffusion of the first base metal X can beshortened, which leads to a reduction in cost.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory section view showing a composite body prior toundergoing extrusion in an embodiment of a superconducting wireaccording to a first aspect of the present invention;

FIG. 2 is an explanatory section view of a superconducting wire in anembodiment according to the first aspect of the present invention;

FIG. 3 is an explanatory section view showing a composite body prior toundergoing extrusion in another embodiment of a superconducting wireaccording to the first aspect of the present invention;

FIG. 4 is an explanatory partial section view showing a superconductingwire in an embodiment according to the first aspect of the presentinvention;

FIG. 5 is a perspective view showing a composite body prior toundergoing extrusion in an embodiment according to the first aspect ofthe present invention;

FIG. 6 is an explanatory section view of a superconducting wire in anembodiment according to the first aspect of the present invention;

FIG. 7 is an explanatory section view showing a composite body prior toundergoing extrusion in an embodiment of a superconducting wireaccording to the first aspect of the present invention;

FIG. 8 is an explanatory partial section view showing a superconductingwire in an embodiment according to the first aspect of the presentinvention;

FIG. 9 is an explanatory section view of a superconducting wireprecursor in an embodiment according to a second aspect of the presentinvention;

FIG. 10 is an explanatory section view of a compound superconductingwire in an embodiment according to the second aspect of the presentinvention;

FIG. 11 is an explanatory view showing an embodiment of a compositesingle core wire for use in manufacturing a superconducting wireprecursor according to the first or second aspect of the presentinvention;

FIG. 12 is an explanatory section view showing another embodiment of asuperconducting wire precursor according to the second aspect of thepresent invention;

FIG. 13 is an explanatory section view showing another embodiment of acompound superconducting wire according to the second aspect of thepresent invention;

FIG. 14 is an explanatory section view showing another embodiment of acomposite single core wire for use in manufacturing a superconductingwire precursor according to the second aspect of the present invention;

FIG. 15 is an explanatory section view showing another embodiment of asuperconducting wire precursor according to the second aspect of thepresent invention;

FIG. 16 is an explanatory view showing another embodiment of a compositesingle core wire for use in manufacturing a superconducting wireprecursor according to the second aspect of the present invention;

FIG. 17 is an explanatory section view showing another embodiment of asuperconducting wire precursor according to the second aspect of thepresent invention;

FIG. 18 is an explanatory view showing another embodiment of a compositesingle core wire for use in manufacturing a superconducting wireprecursor according to the second aspect of the present invention;

FIG. 19 is an explanatory section view showing another embodiment of asuperconducting wire precursor according to the second aspect of thepresent invention;

FIG. 20 is an explanatory section view showing another embodiment of acomposite single core wire for use in manufacturing a superconductingwire precursor according to the second aspect of the present invention;

FIG. 21 is an explanatory view showing another embodiment of a compositesingle core wire for use in manufacturing a superconducting wireprecursor according to the second aspect of the present invention;

FIG. 22 is an explanatory view showing another embodiment of a compositesingle core wire for use in manufacturing a superconducting wireprecursor according to the second aspect of the present invention;

FIG. 23 is an explanatory section view of a conventional superconductingwire precursor; and

FIG. 24 is an explanatory section view of a conventional superconductingwire.

DETAILED DESCRIPTION

In the following description on a superconducting wire and a precursorthereof, a case where Nb and Sn are used will be explained forconvenience. However, it should be understood that Nb and Sn can bereplaced with other metals Z and X, for example, V and Ga, respectively,and that a superconducting wire using a compound other than Nb₃ Snproduces substantially the same effects as a superconducting wire usingNb₃ Sn. Among compounds other than Nb₃ Sn, V₃ Ga is particularlypractical.

The term like "A base metal . . ." or "base metal material A" as usedherein means to include any metal mainly composed of a metal A which mayeither be a pure metal A or contain an additive. The expression like "Abase metal . . ." or " base metal material A" is employed because themetal A as a base metal may form an alloy or an intermetallic compoundin combination with another metal.

The first aspect of the present invention will be described in detail.

The Nb-Sn superconducting wire according to the first aspect of thepresent invention has Nb₃ Sn filaments embedded in a bronze layer withsuch a space between adjacent filaments as not to cause their mutualcontact or coupling. To assure such a space between adjacent Nb₃ Snfilaments while allowing Nb₃ Sn filaments as many as possible to beembedded, the precursor is prepared in such a manner that the spacebetween adjacent Nb base metal filaments is made larger in a region inwhich Nb base metal filaments are easy to come into contact with eachother in the heat treatment step, namely a boundary region of an ε-phasebronze layer produced when preheated at a temperature of 300° to 600°C., than in other regions. The superconducting wire precursor havingsuch an arrangement of the Nb filaments is heat-treated.

The preheating is performed, before the heat treatment for turning thesuperconducting wire precursor into a superconducting wire, at atemperature of 300° to 600° C. for diffusing an Sn base metal materiallocated in the center portion of the precursor into a surrounding Cubase metal material to form a bronze matrix of a Cu-Sn alloy. Thepreheating is followed by the heat treatment.

The Nb-Sn superconducting wire is obtained by the so-called "internaldiffusion method". For instance, it is prepared as follows: Single corewires or filaments obtained by drawing Nb base metal rods or filamentseach covered with the Cu base metal material are stuffed in a billet,preferably in a concentrical multi-layer arrangement, and the resultantis extruded to form a composite body. Alternatively, Cu plates eachformed with apertures are laid on top of another to form a block bodyhaving apertures arranged concentrically in layers in the sectionthereof, Nb base metal rods or Nb single core wires are inserted intothe apertures, respectively, and then the resultant is extruded to forma composite body. Thereafter, an aperture is formed in the centerportion of the composite body and then inserted with an Sn base metalrod, and the resultant is drawn to form a superconducting wireprecursor. Finally the precursor is subjected to a heat treatment at atemperature of 300° to 600° C. for a time sufficient to produce bronzeand then at a temperature of 600° to 800° C. for 100 to 200 hours toform the superconducting wire. The superconducting wire precursor canalso be obtained in such a manner that a tubular composite body isformed by tubular extrusion or cold forming of a billet filled withsingle core wires, an Sn base metal rod is inserted in the centralportion of the tubular composite body, and then the resultant is drawnto form a superconducting wire precursor. The Nb base metal filaments inthe Cu base metal material are converted into Nb₃ Sn filaments eitherpartially or completely by the heat treatment of the precursor, thus thesuperconducting wire is obtained in which the Nb₃ Sn filaments areembedded in the Cu base metal material, typically a Cu-Sn bronze matrix.

In the superconducting wire according to the first aspect of the presentinvention, the spacing between adjacent Nb₃ Sn filaments is made largerin the boundary region of an ε-phase bronze layer produced by thereaction occurring in the heat treatment between the Sn base metalcentrally located and the Cu base metal material than in the otherregions by making each filament thin or arranging the filaments sparselyin the boundary region, whereby the Nb₃ Sn filaments in this region areprevented from their mutual contact. Hence, the superconducting wireoffers excellent electrical characteristics from the viewpoint ofhysteresis loss.

The ε-phase bronze layer is one of the phases of Cu-Sn alloy produced ina heating temperature range of 300° to 600° C. upon the heat treatmentfor causing the centrally-located Sn base metal material to diffuse intothe Cu base metal material to form bronze, and means an intermetalliccompound represented by Cu₃ Sn. The ε-phase bronze is rigid and brittleas compared with α-phase bronze. In the heat treatment at a temperatureof 600° to 800° C., the ε-phase once produced disappears because ofcreation of Nb₃ Sn. The ε-phase is formed outwardly from the center. Theboundary between the ε-phase bronze and the Cu base metal materialusually appears at the position spaced apart from the center by adistance about 50% to about 70% of the radius of the superconductingwire precursor (or of the radius of the circle inside a barrier layer ifprovided), namely a distance about 50% to about 70% of the radius of thematrix wherein filaments are embedded, though the distance variesdepending on the amount of Sn. For instance, the boundary of the ε-phasebronze layer produced when the superconducting wire precursor is heatedat 415° C. appears at a distance of 50 to 70% of the radius of thesuperconducting wire precursor from the center thereof. The heattreatment at a temperature of 600° C. or higher causes the ε-phaseinside the circular boundary to convert into β, γ-phase, which in turnexpands over sustantiantially the entire section to finally form theα-phase bronze matrix.

The reason why the Nb₃ Sn filaments tends to come into mutual contactwhen the superconducting wire precursor is heat-treated has not beenmade clear, but can be assumed that chances of their mutual contact areincreased because the volume of each Nb base metal filament is increasedby about 30% when it turns into the Nb₃ Sn filament, the filaments arelikely to move during the heating, or a like phenomenon occurs. Themovement of the filaments is particularly conspicuous in the boundaryregion of the ε-phase bronze layer.

To avoid mutual contact of the Nb₃ Sn filaments in the ε-phase bronzelayer boundary region, the spacing between adjacent Nb base metalfilaments in the precursor prior to the heat treatment thereof ispreferably at least 0.45 times the diameter of each Nb base metalfilament, especially 0.48 times or more. When the spacing, which isfound experimentally, is less than the above value, many mutual contactsof the filaments are likely to occur though the Jc value of thesuperconducting wire obtained by the heat treatment is expected toincrease. Consequently the resulting superconducting wire suffers anincreased d_(eff) or AC loss and is, therefore, unsuitable for use as asuperconducting wire for pulse current. If the spacing is, for example,0.38 times the diameter of the Nb filament, the d_(eff) value assumes500 μm, resulting in a wire unusable as the superconducting wire forpulse current.

The region in which the spacing between adjacent Nb base metal filamentsneeds to be enlarged is a region near the boundary of the ε-phase bronzelayer produced when heat-treated, preferably a region defined betweencircles having radiuses of 0.7 time and 1.4 times the distance from thecenter of the precursor to the boundary of the ε-phase bronze layerproduced when the precursor is heated at 415° C., especially a regionbetween. 0.9 time and 1.2 times the radius of the circular ε-phasebronze layer. If the spacing between adjacent filaments is enlarged in aregion beyond this range, the number of Nb base metal material isdecreased and, as a result, the Jc value of the superconducting wirefinally obtained is undesirably decreased. The distance between thecenter and the boundary can be found from a photograph of a polishedsection of the superconducting wire taken by using an optical microscopeor electron microscope.

The superconducting wire precursor according to the first aspect of thepresent invention is prepared by, for example, the following methods.

(1) There are prepared two types of single core wires each of which anNb wire is embedded centrally of a Cu base metal material and which havedifferent Cu wall thicknesses. In the center portion of a Cu billet isinserted a Cu base metal rod, and the single core wires are stuffedaround the center Cu rod. At this time, the aforesaid single core wireshaving a thick Cu wall are arranged in the region corresponding to theboundary region of the ε-phase bronze layer produced in the heattreatment for obtaining a superconducting wire, while single core wireshaving a thin Cu wall are arranged in the regions located in the insideand outside of the annulus defined by such a region, or in only theregion located in the outside of the annular boundary region.

The aforesaid two types of single core wires are formed by inserting Nbrods having the same thickness into two sorts of Cu pipes different inwall thickness, respectively and drawing the resultants.

In this case, if the two types of the Cu pipes inserted with Nb wiresare drawn so as to have the same diameter (if the section is polygon,the diameter is meant by the distance across flats), the Nb wire in theobtained core wire having a larger Cu wall thickness becomes thinnerthan in other type of single core wire. Accordingly, when the singlecore wires of the type having larger Cu wall thickness are arranged inthe boundary region of the ε-phase bronze layer while arranging thesingle core wires of the other type in other regions, the spacingbetween adjacent Nb base metal filaments in the boundary region of theε-phase bronze layer is made larger than in other regions.

Alternatively, if the two types of the Cu pipes inserted with the Nbwires are drawn so that the Cu-thick type wires would have a largerdiameter than the Cu-thin type wires, or drawn in the same drawingratio, the spacing between adjacent Nb base metal filaments in theportion filled with the Cu-thick type single core wires can be madelarger, depending on the Cu thickness, than the spacing in the portionfilled with the other type single core wires.

Consequently, in either case, the spacing between adjacent Nb base metalfilaments can be made larger in such a manner.

Thereafter, the Cu billet stuffed with the aforesaid single core wiresand the Cu base metal rod or rods is subjected to an elongation process,e.g., extrusion, to form a composite body, an aperture is formed in thecentral Cu rod portion, and an Sn base metal rod is inserted into theaperture.

Instead of forming an aperture in the center portion of the compositebody, such a Cu billet may be formed into a tubular composite body by atubular extrusion, cold forming or a like technique, which is theninserted with the Sn base metal material in its hollow portion.

Thereafter, the composite body is, as occasion demands, covered with abarrier material against Sn diffusion, for example, Ta, and further witha stabilizing material, and is drawn to form the superconducting wireprecursor. The precursor is then subjected to a preheating at atemperature of 300° to 600° C. to diffuse the centrally-located Sn basemetal material into the Cu base metal surrounding each Nb filament,thereby producing bronze. Further, the resultant is subjected to a heattreatment at a temperature of 600° to 800° C. to provide thesuperconducting wire.

The aforesaid stabilizing material may be provided for forming a layerwhich will not be turned into bronze even by the heat treatment. Ingeneral the stabilizing layer is provided as the outermost layer, butits location is not limited to the outermost portion of the wire and itmay be provided in the inner portion of the superconducting wire. Theprovision of the stabilizing material layer allows to obtain asuperconducting wire which is more stable against electrical and thermaltreatments. The stabilizing material includes Cu, Al having high purity,or the like. To avoid such bronzing, it is advantageous that a barrierlayer for blocking Sn diffusion is provided usually between the Sn-Cucomposite body and the stabilizing layer. The material for the barrierlayer is preferably Ta, but Nb, V or the like is also usable.

(2) In stuffing the single core wires into the Cu billet, the regioninside the ε-phase bronze layer boundary is stuffed with Cu base metalmaterial rods or wires solely, the region immediately outside theboundary is stuffed with the single core wires having a thick Cu wall inat least one layer while the further outer region with single core wireshaving a thin Cu wall. Thereafter, the resultant is processed in amanner similar to that in the method (1).

(3) Oxygen-free copper disk plates each having a multiplicity ofapertures are stuffed in a stacked fashion in the Cu billet, and then Nbrods or wires are inserted into the apertures. The location of aperturesin the oxygen-free copper disk corresponds to that of the single corewires stuffed in the case of the method (1). The spacing betweenadjacent apertures in the region corresponding to the ε-phase bronzelayer boundary region is made larger than in the other regions.Thereafter, the processing is conducted in a manner similar to that inmethod (1).

The outer surface of the composite body as obtained in the above methodsmay in advance be plated with Sn so as to reduce the amount of Sn to beprovided in the center portion.

In order to provide a superconducting wire having a heavy currentcapacity, a plurality of the thus obtained precursor wires may be packedin a Cu pipe and drawn to give a multi-module precursor wire.

In the superconducting wire according to the first aspect of the presentinvention, like that according to the second aspect to be describedlater, the Nb₃ Sn filament may contain as a trace element 0.01 to 5% byweight of at least one element selected from the group consisting of Ti,Ta, Hf, In, Ge, Si, Ga, Mo, Zr, V and Mn. Effects obtained by theinclusion of such a trace element are the same for both the first andsecond aspects of the invention.

Nb₃ Sn containing a trace amount of the above element is obtained by thefollowing methods.

(i) Employed is an Sn alloy containing 0.01 to 10% by weight of at leastone element selected from the group consisting of Ti, In, Ga, Ge, Si andMn, or an Sn molding formed from a metal powder mixture containing 0.01to 10% by weight of at least one of these metals. If the amount of thetrace element is less than 0.01% by weight, any effect cannot berecognized, while, on the other side, if it exceeds 10% by weight, theamount of Sn to be supplied is decreased thereby decreasing the amountof Nb₃ Sn to be produced, resulting in a superconducting wire ofdegraded characteristics.

(ii) Employed is an Nb alloy containing 0.01 to 5% by weight of at leastone element selected from the group consisting of. Ti, Ta, Hf, Mo, Zrand V, or an Nb molded product formed from a metal powder mixturecontaining 0.01 to 5% by weight of at least one of these metals. If theamount of the trace element to be added is less than 0.01% by weight,any effect cannot be recognized, while, on the other side, if it exceeds5% by weight, the amount of Nb to be supplied is decreased therebydecreasing the amount of Nb₃ Sn to be produced, resulting in asuperconducting wire of degraded characteristics.

(iii) Employed is Cu containing 0.01 to 5% by weight of at least oneelement selected from the group consisting of Ti, In, Ge, Si and Mn. Ifthe amount of the trace element to be added is less than 0.01% byweight, any effect cannot be expected, while, on the other side, if itexceeds 5% by weight, the processability is remarkably decreased. Theprocessability can be enhanced by incorporating 0.01 to 1% by weight ofSn in the Cu base metal material.

The mutual contact of the Nb₃ Sn filaments can be decreased by notarranging the filaments in the inside of the boundary of the ε-phasebronze layer with or without increasing the spacing between any adjacentfilaments arranged in the boundary region of the ε-phase bronze layer.Accordingly, in another embodiment of the present invention, Nb-Sncompound superconducting wires having improved characteristics areobtained by arranging Nb base metal filaments separately from each otherin a continuous phase of a Cu base metal material, in the center portionof which an Sn base metal material is arranged, according to aconventional internal diffusion method so that the Nb base metalfilaments are present only in the outside of the boundary of the ε-phasebronze layer, and then drawing the resultant to form a precursor wireand heat-treating the precursor wire. In the thus prepared precursorwire, when the spacing between any adjacent Nb base metal filamentslocated in the boundary region of the ε-phase bronze layer is madelarger than the spacing in the other portion in the manner describedabove, further improved superconducting wires are obtained.

The superconducting wire precursor according to the second aspect of thepresent invention is obtained by the following methods.

(I) Cu base metal plates and Sn base metal plates are alternatelystacked and then rolled to form an integrally-formed Cu-Sn compositebody. The resulting composite body is stuffed in an oxygen-free coppercontainer along the length thereof, and the resultant is extruded toform a Cu-Sn composite rod. A plurality of apertures are formed in thecomposite rod in the longitudinal direction thereof, Nb base metal rodsare inserted into the respective apertures, and then the resultant isdrawn.

(II) A multiplicity of composite single core wires each obtained bycovering an Nb base metal rod with a composite of Cu and Sn are tightlystuffed in an oxygen-free copper container, and then this container isdrawn.

Preferably, the container prior to the drawing is covered with a barriermaterial against Sn and further with a pipe of a stabilizing material,as occasion demands, followed by drawing. However, it is not alwaysnecessary to provide the stabilizing layer as the outermost layer, andit may be arranged in a desired location according to design of thewire. There is preferred a superconducting wire precursor having astabilizing layer of Cu or Al free of Sn. The barrier material is thosecapable of preventing Sn diffused during the heat treatment fromreaching the layer of the stabilizing material.

The proportion of Sn in the aforesaid composite of Cu and Sn is from 1to 99% by weight, preferably 13 to 20% by weight. The amount of Snsmaller than 1% by weight is insufficient and thus a sufficient amountof Nb₃ Sn is difficult to produce. When the amount of Sn exceeds 99% byweight, the volume rate of Sn is increased too much, thus rendering thecomposite too soft to be processed.

The aforesaid composite single core wire used in method (II) is preparedby the following methods.

(a) An Nb base metal rod is inserted into a Cu base metal pipe, theresultant is extruded to form a single core wire, and the outer surfaceof the single core wire is plated with Sn to form the composite singlecore wire.

(b) A plurality of apertures are formed in the wall of a tubularoxygen-free copper container in the longitudinal direction, and intoeach of the apertures are inserted Sn rods. An Nb base metal rod isinserted into the central portion of the container, followed byextrusion to form the composite single core wire.

(c) An Sn plate is sandwiched between two Cu plates and the resultant isrolled to form an integrated Cu-Sn composite plate. Otherwise, a Cuplate is plated with Sn on at least one side thereof. The Cu-Sncomposite plate or Sn-plated Cu plate is wrapped around an Nb base metalrod several times, and the resultant is extruded to form the compositesingle core wire.

In the superconducting wire precursor according to the second aspect ofthe present invention, the spacing between adjacent Nb base metalfilaments needs to be 14/100 or more, especially 30/100 or more, of thediameter of a single filament so as to avoid mutual contact thereof.However, if the spacing is too large, the number of Nb₃ Sn filamentsaccommodated in the superconducting wire is undesirably reduced,resulting in a decreased Jc value. On the other hand, too small spacingcauses the d_(eff) value to increase. Therefore, it is desirable thatthe spacing between adjacent filaments is larger than the above valuebut as small as possible.

The superconducting wire according to the second aspect of the presentinvention is obtained by preheating the superconducting wire precursorat a temperature ranging from 200° to 600° C. for an appropriatelyselected time period to convert the Cu-Sn composite into bronze, thenheating it at a temperature of 600° to 800° C. for about 100 to 200hours to convert the Nb base metal filaments into Nb₃ Sn filaments. Theheat treatment at a temperature of 600° to 800° C. can be made afteronce cooling the preheated precursor.

The aforesaid Nb₃ Sn filaments may each contain as a trace element 0.01to 5% by weight of at least one element selected from the groupconsisting of Ti, Ta, Hf, In, Ge, Si, Ga, Mo, Zr, V and Mm.

The use of the Nb₃ Sn filament containing a trace amount of at least oneelement selected from Ti, Ta, Hf, Mo, Zr and V improves the Jc value inhigher magnetic field. When In or Ga is used, the Jc value inintermediate or lower magnetic field is improved with improvedworkability of wire. The addition of Ge, Si or Mn is effective inreducing AC loss.

Nb₃ Sn containing the above trace element is obtained by the followingmethods.

(i) Employed is Sn containing 0.01 to 10% by weight of at least oneelement selected from the group consisting of Ti, In, Ga, Ge, Si and Mn.If the content of the element is less than 0.01% by weight, any effectcannot be recognized, while if it exceeds 10% by weight, the amount ofSn to be supplied is decreased thereby undesirably reducing the amountof Nb₃ Sn, resulting in degradation in characterisctis ofsuperconducting wire.

(ii) Employed is Nb containing 0.01 to 5% by weight of at least oneelement selected from the group consisting of Ti, Ta, Hf, Mo, Zr and V.If the content of the element is less than 0.01% by weight, any effectcannot be recognized. If it exceeds 5% by weight, the amount of Nb isdecreased, thus the amount of Nb₃ Sn to be finally produced isundesirably reduced, resulting in deterioration in characteristics.

(iii) Employed is Cu containing 0.01 to 5% by weight of at least oneelement selected from the group consisting of Ti, In, Ge, Si and Mn. Ifthe content of the element is less than 0.01% by weight, any effectcannot be recognized, while if it exceeds 5% by weight, substantiallypoor workability will result.

The workability can be improved by incorporating 0.01 to 1% by weight ofSn into the Cu base metal material.

(iv) A composite body of Cu base metal material and Sn base metalmaterial is plated on its surface with at least one element selectedfrom the group consisting of Ti, In, Ge, Si, Mn, Ni and Sn.

In the case of Ti, a stack of a thin plate of Ti on the aforesaidcomposite body may be used.

(v) The Nb base metal material is plated on its surface with at leastone element selected from the group consisting of Ti, Ta, Hf, Mo, Zr andV.

Hereinafter, the method for manufacturing the superconducting wireaccording to the present invention will be specifically described withreference to the drawings. It should be understood that Examples 1 to 4are associated with the first aspect of the invention and Examples 5 to12 associated with the second aspect of the invention.

EXAMPLE 1

FIG. 1 is an explanatory section showing a composite body prior toundergoing extrusion which is incorporated in a Cu billet. In FIG. 1,composite body 1 comprises a billet 2, Nb single core wires 13a and 13bto be described later, and wires 4a of Cu base metal material(hereinafter referred to as "Cu base metal wire").

First, two types of Nb single core wires 13a and 13b were formed whichwould be Nb base metal filaments in a superconducting wire precursor.Specifically, an Nb base metal rod having a diameter of 11 mm in theform of round bar was inserted into a Cu base metal pipe having innerand outer diameters of 11.8 mm and 16.8 mm, and the resultant was drawnto form a hexagonal wire (denoted by numeral 13a in FIG. 1) of 4.2 mmacross flats. In the same manner, the single core wire 13b (4.2 mmacross flats) of which Cu wall thickness was larger than that of thesingle core wire 13a was formed from an Nb base metal rod of 11 mmdiameter in the form of round bar and a Cu base metal pipe of 11.8 mminner diameter and 18.4 mm outer diameter.

The Nb single core wires 13a and 13b and hexagonal Cu base metal wire 4aof 4.2 mm across flats were stuffed in a billet with an arrangementwherein radially from the center of the billet were disposed sevenlayers of Cu base metal wire 4a, two layers of single core wire 13a,three layers of single core wire 13b, and five layers of single corewire 13a. For simplicity, FIG. 1 depicts only a single line drawingradially. The reason why such an arrangement was employed is as follows.In the production of ε-phase bronze layer by formation of an alloy fromthe Cu base metal material and a centrally located Sn base metalmaterial by heat treatment, since the boundary of the ε-phase bronzelayer appears between the third and fourth layers of Nb single corewires from the center, the diameter of each Nb base metal filament inthe third to fifth layers is made smaller than in other layers toslightly enlarge the spacing between adjacent filaments after drawnthereby avoiding mutual contact of Nb₃ Sn filaments to be produced by aheat treatment.

Thereafter, the billet was subjected to extrusion, the extrudedcomposite body was centrally drilled, and an Sn base metal rod wasinserted into the drilled aperture, followed by drawing the resultant toobtain a composite wire. This composite wire was inserted into a Ta pipeserving as a barrier material against Sn diffusion. This pipe wasfurther covered with a Cu pipe for stabilization to achieve secondarycomposite, followed by being drawn to have a wire diameter of 0.5 mm.

The superconducting wire precursor was subjected to a preheating then toa heat treatment to produce Nb₃ Sn in the Nb base metal filamentportion, yielding an Nb-Sn superconducting wire. The temperature andperiod of time for this heat treatment need to be those causing athermal diffusion reaction and forming a superconductor. In the case ofNb₃ Sn, the heat treatment was performed at 600° to 800° C. for 100 to200 hours.

FIG. 2 is an explanatory section of the thus manufacturedsuperconducting wire having been heat-treated, and wherein denoted atnumeral 9 is the superconducting wire after undergoing heat treatment,at numeral 10 Nb₃ Sn filament, at numeral 11 low-Sn-concentrationbronze, at numeral 7 barrier layer of Ta, and at numeral 8 stabilizinglayer of Cu.

Nb base metal filaments in the ε-phase bronze layer boundary region werethinner to some extent than in other regions and, hence, the spacebetween adjacent filaments was made slightly larger than in otherregions.

The aforesaid low-Sn-concentration bronze means a bronze of anSn-concentration (about 3% to 10% by weight) lower than that (about 18%to 20% by weight) of bronze once produced during the heat treatment. Thereason why the Sn concentration of bronze is lowered is that Sn isfurther diffused by the heat treatment to produce Nb₃ Sn. Thislow-Sn-concentration bronze region substantially corresponds to theregion extending from the center of the superconducting wire to the Nbfilament region.

The superconducting wire thus obtained was measured for Jc and d_(eff)in liquid helium. The measurement revealed that the Jc value was 820A/mm² in a magnetic field of B=12 T. In terms of Jc characteristic thevalue found was reduced by 5% as compared with that of a conventionaltypical superconducting wire since the space factor of Nb₃ Sn wasdecreased. However, there was found a value of 9 μm in d_(eff), whichwas about 1/4 of the typical superconducting wire. When the twosuperconducting wires are totally estimated in terms of Jc/d_(eff), thesuperconducting wire of the present invention is found to have beenimproved 3.8 times the typical one.

EXAMPLE 2

FIG. 3 is an explanatory section of another embodiment of a compositebody prior to undergoing extrusion which is incorporated in a Cu billet,and wherein numeral 4b denotes Cu base metal rod.

First, two types of Nb single core wires 13c and 13d were formed whichwould be Nb base metal filaments in a superconducting wire precursor.Specifically, an Nb base metal rod having a diameter of 11 mm in theform of round bar was inserted into a Cu base metal pipe having innerand outer diameters of 11.8 mm and 16.8 mm, and the resultant was drawnto have a wire diameter of 4.2 mm and to form the single core wire 13cwith a small Cu thickness. Similarly, the single core wire 13d having awire diameter of 4.6 mm with a large Cu thickness was formed byinserting an Nb base metal rod of 11 mm diameter in the form of roundbar into a Cu base metal pipe of 11.8 mm inner diameter and 18.4 mmouter diameter, and drawing the resultant. In this Example, by using twotypes of single core wires different in thickness formed by making Cuthickenss different, the space between adjacent filaments was enlargedin the ε-phase bronze layer boundary region in which mutual contact offilaments was particularly likely.

As shown in FIG. 3, a billet was stuffed radially from the centerthereof with Cu base metal rod 4b of 20 mm diameter, two layers ofsingle core wire 13c, three layers of single core wire 13d, and fourlayers of single core wire 13c (some single core wires are omitted inFIG. 3). The reason why such an arrangement was employed is the same asin Example 1. The clearances formed by thus stuffing single core wiresin the form of round bar were inserted with thin Cu base metal wires(omitted in FIG. 3), and the resultant was extruded to form a compositebody. Thereafter, the composite body thus extruded was centrally drilledto form an aperture into which an Sn base metal rod was then inserted,and the resultant was further drawn to form a composite wire. Thiscomposite wire was inserted into a Ta pipe serving as a barrier materialagainst Sn diffusion and further into a Cu pipe for stabilization toachieve a secondary composition. The resultant was drawn to have a wirediameter of 0.5 mm to yield a superconducting wire precursor.

The superconducting wire precursor was subjected to a preheating then toa heat treatment at 600° to 750° C. for 100 to 200 hours to yield anNb-Sn superconducting wire.

FIG. 4 is an explanatory section of the thus manufacturedsuperconducting wire having been heat-treated, and wherein denoted atnumeral 9 is the superconducting wire after undergoing heat treatment,at numeral 10 Nb₃ Sn filament, at numeral 11 low-Sn-concentration bronzelayer, at numeral 7 barrier layer of Ta, and at numeral 8 stabilizingmaterial of Cu.

The superconducting wire thus obtained was measured for Jc and d_(eff)in liquid helium. The measurement revealed that the Jc value of thissuperconducting wire was 805 A/mm² in a magnetic field of B=12 T and thed_(eff) value was 6 μm. When the superconducting wire thus obtained iscompared with a conventional one by totally estimating these in terms ofJc/d_(eff), the superconducting wire of the present invention is foundto have been improved 5.6 times the conventional one.

EXAMPLE 3

FIG. 5 is a perspective view showing a composite body prior toundergoing extrusion which has been incorporated in a Cu billet, andwherein denoted at numeral 1 is the composite body prior to undergoingextrusion, at numeral 2 the Cu billet, at numeral 13 Nb base metal rodsbecoming filaments of superconducting wire precursor, and at numeral 4cCu base metal material in the form of disk.

The Cu base metal material 4c in the form of disk was formed in thefollowing manner. 309 apertures of 4.95 mm diameter were formed in anoxygen-free copper disk of 160 mm diameter and 10 mm thickness using anNC drilling machine. In this case, apertures between the innermost firstand second layers (counted radially) were arranged closely to eachother, those between the second and fourth layers sparsely, and thosebetween the fourth and outermost fifth layers closely. 30 pieces of theCu base metal material disk 4c were stuffed in a billet container ofoxygen-free copper of 180 mm outer diameter and 160 mm inner diameterwith their apertures registered with each other. Nb base metal rods 13of 4.9 mm diameter were inserted into the apertures of the stacked Cudisks, respectively. Finally, air in the billet was evacuated, and acover was welded to the billet to form a composite billet. Insertion ofthe Nb base metal rods into the oxygen-free copper disks in stacks wasconducted with ease.

The composite billet was extruded and centrally formed with an apertureinto which an Sn base metal rod was then inserted. The resultant wasdrawn to form a composite wire. The composite wire was inserted in a Tapipe serving as a barrier material against Sn diffusion, and theresultant was further inserted into a Cu pipe to achieve secondarycomposition, followed by being drawn to have a wire diameter of 0.5 mm.

The superconducting wire precursor thus formed was subjected to apreheating then to a heat treatment at 600° to 750° C. for 100 to 200hours to yield an Nb-Sn superconducting wire.

FIG. 6 is an explanatory section of the thus obtained superconductingwire having been heat-treated, and wherein denoted at numeral 9 is thesuperconducting wire having been heat treated, at numeral 10 Nb₃ Snfilament, at numeral 11 low-Sn-concentration bronze, at numeral 7 abarrier layer of Ta, and at numeral 8 a stabilizing layer of Cu.

The superconducting wire thus obtained was measured for Jc and d_(eff)in liquid helium. The measurement revealed that the Jc value of thissuperconducting wire was 930 A/mm² in a magnetic field of B=12T and thed_(eff) value was 6 μm. When the superconducting wire thus obtained iscompared with a conventional one by totally estimating these in terms ofJc/d_(eff), the superconducting wire of the present invention is foundto have been improved 5.6 times the conventional one.

EXAMPLE 4

A superconducting wire may be required to be improved in Jccharacteristic more than in d_(eff) characteristic. To meet thisrequirement, as a variation of the embodiment of Example 1 such anarrangement is offered in this Example that single core wires aredisposed so that Nb base metal filaments would exist only in the regionoutside the boundary of the aforementioned ε-phase bronze layer whenε-phase bronze layer is produced during the heat treatment. Thisarrangement avoids Nb₃ Sn filaments from coming into mutual contact orcoupling while effectively reducing the d_(eff) value.

FIG. 7 is an explanatory section showing a composite body prior toundergoing extrusion which is incorporated in Cu billet. In FIG. 7,composite body 1 comprises Cu billet 2, hexagonal Nb single core wires13e and 13f, and hexagonal Cu base metal wire 4a of 4.2 mm across flats.To make the space between adjacent filaments larger than in theembodiment of Example 1, the single core wires 13e and 13f were formedby inserting Nb base metal rods in the form of round bar into a Cu pipeof 11.8 mm inner diameter and 17.4 mm outer diameter for 13e and a Cupipe of 11.8 mm inner diameter and 19.3 mm outer diameter for 13f,respectively and drawing the resultants to form hexagonal single corewires of 4.2 mm across flats.

The Nb single core wires and Cu base metal wire a were stuffed in abillet with an arragement wherein radially from the center were disposedseven layers of Cu base metal wire 4a, three layers of single core wire13f, and seven layers of single core wire 13e. For simplicity, FIG. 7depicts only a single line of single core wires 13e and 13f and Cu basemetal wire 4a which is radially drawing though these wires are stuffedin the overall section. The resulting billet was subjected to extrusion,the extruded composite body was centrally formed with an aperture, andan Sn alloy rod of 14 mm diameter containing 1.5% by weight of Ti wasinserte into the aperture, followed by drawing the resultant to form acomposite wire. The composite wire is circumferentially plated with Snto about 30 μm thickness, inserted into a Ta pipe serving as a barriermaterial against Sn diffusion, and further inserted into a Cu pipe forstabilization to achieve secondary composition, followed by being drawnto have a wire diameter of 0.5 mm. The reason why Sn plating wasconducted is that by diffusing Sn to the outer portion the amount of theSn base metal material to be centrally located is reduced therebyminimizing the area of the ε-phase bronze layer to be produced by thecentrally-located Sn.

The superconducting wire precursor was subjected to a preheating andthen to a heat treatment at 600° C. to 800° C. for 100 to 200 hours toyield an Nb-Sn superconducting wire. The temperature for this heattreatment needs to be a temperature causing a thermal diffusionreaction, thus forming a superconductor. In the case of Nb₃ Sn, the heattreatment temperature is from 600° to 800° C.

FIG. 8 is an explanatory section of the thus manufacturedsuperconducting wire having been heat-treated, and wherein denoted atnumeral 9 is the superconducting wire after undergoing the heattreatment, at numeral 10 Nb₃ Sn filament, at numeral 11low-Sn-concentration bronze, at numeral 7 a barrier layer of Ta, and atnumeral 8 a stabilizing layer of Cu.

The superconducting wire thus obtained was measured for Jc and d_(eff)in liquid helium. The measurement revealed that the Jc value was 720A/mm² in a magnetic field of B=12T. In terms of Jc characteristic thevalue found was reduced by 16% as compared with that of the conventionalsuperconducting wire since the space factor of Nb₃ Sn was decreased.However, there was found a value of 4 μm in d_(eff), which was about 1/9as large as that (36 μm) of the conventional superconducting wire. Thisvalue is superior even to those of the embodiments of Examples 1 to 3.When this superconducting wire is compared with the conventional one bytotal estimation in terms of Jc/d_(eff), the superconducting wire of thepresent invention is found to have been improved 7.5 times theconventional one.

It should be understood that although the superconducting wires offoregoing Examples 1 to 4 each have a stabilizing layer of Cu and adiffusion barrier layer of Ta, no problem will result even if theselayers are omitted.

EXAMPLE 5

FIG. 9 is an explanatory section showing a superconducting wireprecursor according to the second aspect of the present invention, whichhas been reduced in section area by drawing and has not yet beensubjected to a heat treatment. In FIG. 9, the superconducting wireprecursor 21 comprises a composite body 22 of Cu base metal material andSn base metal material, and Nb base metal filaments 23. FIG. 10 is anexplanatory section showing a superconducting wire obtained bysubjecting the superconducting wire precursor to the heat treatment. Thesuperconducting wire 24 in FIG. 10 comprises Nb₃ Sn filaments 25, andbronze layer 26 of which Sn concentration is lowered by a reaction of Nbwith Sn diffused by the heat treatment.

The composite body 22 of the Cu base metal material and Sn base metalmaterial was formed in the following manner. First, thick plates of 15mm thickness each resulting from rolling and integrating ten oxygen-freecopper plates of 2 mm thickness and nine Sn plates of 1 mm thickness inan alternately stacked fashion were tightly stuffed in an oxygen-freecopper container of 180 mm outer diameter and 160 mm inner diameteralong the length thereof. Next, air in the container was evacuated, acover was welded to the container, and the resultant was subjected tocold hydrostatic extrusion to form a composite in the form of a rod of90 mm diameter. Finally, the oxygen-free copper in an outer peripheralportion of this composite rod was eliminated by external cutting using alathe to form a columnar composite 22 of 80 mm diameter.

The composite 22 was cut to a length of 100 mm and then drilled to form127 apertures of 4.1 mm diameter. Next, Nb base metal wires of 4.0 mmdiameter and 100 mm length were individually inserted into the aperturesof the composite, and the resultant was accommodated in an oxygen-freecopper container of 90 mm outer diameter and 80 mm inner diameter. Airin the container was evacuated, and a cover was welded to the containerby electron beam welding to form a composite billet. The compositebillet was subjected to cold hydrostatic extrusion and then eliminatedof oxygen-free copper in an outer peripheral portion thereof by externalcutting using a lathe. The resultant was drawn to have a final diameterand then twisted to form a superconducting wire precursor of 0.2 wirediameter. The thus obtained superconducting wire precursor was subjectedto a preheating then to a heat treatment at 600° to 800° C. for 50 to200 hours to form a superconducting wire having Nb₃ Sn superconductorproduced in the Nb base filament portion.

The superconducting wire thus obtained was measured for Jc and d_(eff)in liquid helium. The measurement revealed that the Jc value of thissuperconducting wire was 550 A/mm² in a magnetic field of B=12 T and thed_(eff) value was 5 μm. When the superconducting wire thus obtained iscompared with an Nb₃ Sn superconducting wire obtained by the prior artinternal diffusion method by totally estimating the two in terms ofJc/d_(eff), this superconducting wire of the present invention is foundto have been improved 4.4 times the prior art superconducting wire.

EXAMPLE 6

FIG. 11 is an explanatory view showing one embodiment of a compositesingle core wire which will provide an Nb base metal filament in asuperconducting wire precursor according to the second aspect of thepresent invention. The composite single core wire comprises Nb basemetal rod 23b containing 1% by weight of Ti, Cu base metal material 28,and Sn plated layer 29 covering the Cu base metal material.

This composite single core wire was formed in the following manner. AnNb base metal rod of 11 mm diameter incorporated with 1% by weight of Tiwas inserted into a Cu pipe of 11.8 mm inner diameter and 18.4 mm outerdiameter, and the resultant was drawn to form a hexagonal single corewire of 4.2 mm across flats. The single core wire was covered with anSn-plated layer 29 of about 100 μm thickness to yield the compositesingle core wire shown in FIG. 11.

An oxygen-free copper billet of 180 mm outer diameter and 160 mm innerdiameter was stuffed with 1225 pieces of the composite single core wirethus formed, air in the billet was evacuated, and a cover was welded tothe billet by electron beam welding to form a composite billet. Thecomposite billet was subjected to cold hydrostatic extrusion and drawnto form a composite body. The resultant composite body was inserted intoa Ta pipe serving as a barrier material against Sn diffusion, andfurther inserted into a Cu pipe for stabilization to achieve secondarycomposition, followed by being drawn to have a final wire diameter. Thecomposite body having the final diameter was twisted to yield asuperconducting wire precursor of 0.3 mm wire diameter.

FIG. 12 is an explanatory section of the superconducting wire precursorthus obtained, and wherein denoted at numeral 21 is the superconductingwire precursor, at numeral 23b Nb base metal filament containing 1% byweight of Ti, at numeral 30 Ta barrier layer, at numeral 31 Cu layer forstabilization, at numeral 32 Cu base metal material, and at numeral 33Sn-plated layer on the surface of the Cu base metal material.

The thus obtained superconducting wire precursor was subjected to apreheating then to a heat treatment at 600° to 800° C. for 50 to 200hours to form a superconducting wire having Nb₃ Sn superconductorproduced in the Nb base filament portion. FIG. 13 is an explanatorysection of the superconducting wire thus obtained, and wherein thesuperconducting wire 24 comprises Nb₃ Sn filaments 25,low-Sn-concentration bronze 26, barrier layer 30 of Ta, and Cu layer 31for stabilization.

The superconducting wire thus obtained was measured for Jc and d_(eff)in liquid helium. The measurement revealed that the Jc value of thissuperconducting wire was 850 A/mm² in a magnetic field of B=12T and thed_(eff) value was 3 μm. When the superconducting wire thus obtained iscompared with an Nb₃ Sn superconducting wire containing Ti and obtainedby the prior art internal diffusion method by totally estimating the twoin terms of Jc/d_(eff), this superconducting wire of the presentinvention is found to have been improved 7.1 times the prior artsuperconducting wire.

EXAMPLE 7

FIG. 14 is an explanatory view showing one embodiment of a compositesingle core wire prior to being reduced in area for use in manufacturinga superconducting wire precursor according to the second aspect of thepresent invention. The composite single core wire comprises Cu basemetal material 32, Nb base metal rod 23a, and Sn base metal rods 34containing In in an amount of 7% by weight. Such a composite single corewire was formed in the following manner.

First, an Nb base metal rod of 13.9 mm diameter was centrally insertedinto an oxygen-free copper billet container of 25 mm outer diameter and14 mm inner diameter having eight apertures of 3.5 mm diameter as shownin FIG. 14. Eight Sn base metal rods of 3.4 mm diameter containing 7% byweight of In were individually inserted into the apertures lying aroundthe center of the container. Air in the billet container was evacuated,a cover was welded to the billet container by electron beam welding. Theresultant container was subjected to cold hydrostatic extrusion and thendrawn to form a hexagonal composite single core wire of 4.2 mm acrossflats.

A Cu billet similar to that in Example 6 was stuffed with 1225 pieces ofthe single core wire thus formed, air in the billet was evacuated, and acover was welded by electron beam welding to form a composite billet.This composite billet was subjected to cold hydrostatic extrusion andthen drawn to form a composite wire. The resultant composite wire wasinserted into a Ta pipe to serve as a barrier material against Sndiffusion, and further inserted into a Cu pipe for stabilization toachieve secondary composition, followed by being drawn to have a finalwire diameter. The composite wire of the final diameter was twisted toyield a superconducting wire precursor of 0.3 mm wire diameter.

FIG. 15 is an explanatory section of the superconducting wire precursorthus obtained, and wherein superconducting wire precursor 21 comprisesNb base metal filaments 23a, Ta barrier layer 30, Cu layer 31 forstabilization, Cu base metal material 32, and Sn base metal material 34containing 7% by weight of In.

The thus obtained superconducting wire precursor was subjected to apreheating and then to a heat treatment at 600° to 800° C. for 50 to 200hours to form a superconducting wire having Nb₃ Sn superconductorproduced in the Nb base metal filament portion. The sectional structureof the thus obtained superconducting wire was similar to that shown inFIG. 13.

The superconducting wire thus obtained was measured for Jc and d_(eff)in liquid helium. The measurement revealed that the Jc value of thissuperconducting wire was 633 A/mm² in a magnetic field of B=12 T and thed_(eff) value was 3 μm. When the superconducting wire thus obtained iscompared with an Nb₃ Sn superconducting wire containing In and obtainedby the prior art internal diffusion method by totally estimating the twoin terms of Jc/d_(eff), this superconducting wire of the presentinvention is found to have been improved 7.0 times the prior artsuperconducting wire.

EXAMPLE 8

FIG. 16 is an explanatory view showing a constitution of a compositesingle core wire prior to being reduced in section area for use inmanufacturing a superconducting wire according to the second aspect ofthe present invention. The composite single core wire comprisescomposite body 22b of Cu containing 1% by weight of Ti and Sn, and Nbbase metal rod 23a.

The composite body 22b was a thin plate formed by rolling to integrate astack of two Cu plates of 1 mm thickness containing 1% by weight of Tiand an Sn plate of 1 mm thickness sandwiched between the Cu plates. Thecomposite body 22b in the form of thin plate was cut to a size of 140mm×1000 mm and then wound around an Nb base metal rod 23a of 10 mmdiameter and 1000 mm length in the form of round bar. The resultant wasinserted into an oxygen-free copper pipe 35 of 18 mm inner diameter and19 mm outer diameter, followed by drawing to form a hexagonal compositesingle core wire of 4.2 mm across flats.

An oxygen-free copper billet of 180 mm outer diameter and 160 mm innerdiameter was stuffed with 1225 pieces of the single core wire thusformed, air in the billet was evacuated, and a cover was welded byelectron beam welding. This billet was subjected to cold hydrostaticextrusion and then drawn to form a composite wire. The resultantcomposite wire was inserted into a Ta pipe serving as a barrier materialagainst Sn diffusion, and further inserted into a Cu pipe forstabilization to achieve secondary composition, followed by being drawnto have a final wire diameter. The composite wire of the final diameterwas twisted to yield a superconducting wire precursor of 0.3 mmdiameter.

FIG. 17 is an explanatory section of the superconducting wire precursorthus obtained, and wherein superconducting wire precursor comprises acomposite body 22b of Cu containing 1% by weight of Ti and Sn, Nb basemetal filaments 23a, Ta barrier layer 30, Cu layer 31 for stabilization,and Cu base metal material 32.

The thus obtained superconducting wire precursor was subjected to apreheating and then to a heat treatment at 600° to 800° C. for 50 to 200hours to form a superconducting wire having Nb₃ Sn superconductorproduced in the Nb base metal filament portion. The sectional structureof the thus obtained superconducting wire was similar to that shown inFIG. 13.

The superconducting wire thus obtained was measured for Jc and d_(eff)in liquid helium. The measurement revealed that the Jc value of thissupercoducting wire was 900 A/mm² in a magnetic field of B=12 T and thed_(eff) value was 3 μm. When the superconducting wire thus obtained iscompared with an Nb₃ Sn superconducting wire containing Ti and obtainedby the prior art internal diffusion method by totally estimating the twoin terms of Jc/d_(eff), this superconducting wire of the presentinvention is found to have been improved 7.5 times the prior artsuperconducting wire.

EXAMPLE 9

FIG. 18 is an explanatory view showing a constitution of a compositesingle core wire prior to being reduced in section area for use inmanufacturing a superconducting wire according to the second aspect ofthe present invention. The composite single core wire comprisescomposite body 22c of a Cu base metal material plated on its surfacewith Sn, Nb base metal rod 23c plated on its surface with Ti, andoxygen-free copper pipe 35.

An Nb base metal rod of 10 mm diameter which become a filament of asuperconducting wire was electro-coated with a Ti layer of about 45 μmthickness. An oxygen-free copper plate of about 200 mm width and 0.5 mmthickness was plated with an Sn layer of about 0.1 mm to form a Cu-Sncomposite body 22c. The composite body 22c was wound about five timesaround the Ti-plated Nb base metal rod 23c as a core, then inserted intooxygen-free copper pipe 35 of 18 mm inner diameter and 19 mm outerdiameter, followed by being drawn to form a hexagonal composite singlecore wire of 4.2 mm across section.

A Cu billet similar to that used in Example 6 was stuffed with 1225pieces of the composite single core wire thus formed, air in the billetwas evacuated, and a cover was welded to the billet by electron beamwelding. The resultant was then subjected to cold hydrostatic extrusionand drawn to form a composite wire. The composite wire thus formed wasfurther drawn to have a final wire diameter and then twisted to form asuperconducting wire precursor of 0.3 mm wire diameter.

FIG. 19 is an explanatory section of the superconducting wire precursorthus obtained, wherein the superconducting wire precursor comprises Cucomposite body 22c plated on its surface with Sn, Nb base metalfilaments 23c plated on its surface with Ti, Cu layer 31 forstabilization, and Cu base metal material 32.

The thus obtained superconducting wire precursor was subjected to apreheating and then to a heat treatment at 600° to 800° C. for 50 to 200hours to form a superconducting wire having Nb₃ Sn superconductorproduced in the Nb base metal filament portion. The sectional structureof the thus obtained superconducting wire was similar to that shown inFIG. 13.

The superconducting wire thus obtained was measured for Jc and d_(eff)in liquid helium. The measurement revealed that the Jc value of thissupercoducting wire was 900 A/mm² in a magnetic field of B=12T and thed_(eff) value was 3 μm. When the superconducting wire thus obtained iscompared with an Nb₃ Sn superconducting wire containing Ti and obtainedby the prior art internal diffusion method by totally estimating the twoin terms of Jc/d_(eff), this superconducting wire of the presentinvention is found to have been improved 7.5 times the prior artsuperconducting wire.

EXAMPLE 10

FIG. 20 is an explanatory section view showing a constitution of acomposite single core wire prior to being reduced in section area foruse in manufacturing a superconducting wire according to the secondaspect of the present invention. The composite single core wirecomprises Cu-Sn composite body 22c composed of a Cu base metal materialplated on its both sides with Sn, Nb base metal rod 23a, Cu base metalpipe 35, and Ti thin plate 36 sandwiched between the Cu-Sn compositebody 22c and the Nb base metal rod 23a.

Single core wire 1 which becomes a filament of a superconducting wirewas formed in the following manner. In the same manner as in Example 9,an oxygen-free copper plate of about 200 mm width and 0.5 mm thicknesswas plated with Sn in about 0.1 mm thickness to form the Cu-Sn compositebody 22c. Ti thin plate 36 of 0.05 mm thickness and about 30 mm widthwas stacked on the composite body 22c. The composite body 22c with theTi thin plate 36 was wound about five times around the Nb base metal rod23a of 10 mm diameter as a core. Since the Ti thin plate 36 was of onlyabout 30 mm width, it was wound around the Nb base metal rod only in alayer. The resultant was then inserted into oxygen-free copper pipe 35of 18 mm inner diameter and 19 mm outer diameter, followed by beingdrawn to form a hexagonal composite single core wire of 4.2 mm acrosssection.

A Cu billet similar to that used in Example 6 was stuffed with 1225pieces of the composite single core wire thus formed, air in the billetwas evacuated, and a cover was welded to the billet by electron beamwelding. The resultant was then subjected to cold hydrostatic extrusionand drawn to form a composite wire. The composite wire was inserted intoa Ta pipe serving as a barrier material against Sn diffusion and furtherinserted into a Cu pipe for stabilization to achieve secondarycomposition. The secondary composition thus formed was further drawn tohave a final wire diameter and then twisted to form a superconductingwire precursor of 0.3 mm wire diameter.

The thus obtained superconducting wire precursor was subjected to apreheating and then to a heat treatment at 600° to 800° C. for 50 to 200hours to form a superconducting wire having Nb₃ Sn superconductorproduced in the Nb base metal filament portion. The sectional structureof the thus obtained superconducting wire was similar to that shown inFIG. 13.

The superconducting wire thus obtained was measured for Jc and d_(eff)in liquid helium. The measurement revealed that the Jc value of thissupercoducting wire was 900 A/mm² in a magnetic field of B=12T and thed_(eff) value was 3 μm. When the superconducting wire thus obtained iscompared with an Nb₃ Sn superconducting wire containing Ti and obtainedby the prior art internal diffusion method by totally estimating the twoin terms of Jc/d_(eff), this superconducting wire of the presentinvention is found to have been improved 7.5 times the prior artsuperconducting wire.

EXAMPLE 11

FIG. 21 is an explanatory view showing a structure of a composite singlecore wire prior to being reduced in section area for use inmanufacturing a superconducting wire according to the second aspect ofthe present invention. The composite single core wire comprises Cu-Sncomposite body 22d plated on its surface with pure Ti, Nb base metal rod23a, and Cu base metal material pipe 35.

First, in the same manner as in Example 8, a thin plate of 1 mmthickness was formed by rolling to integrate a stack of two Cu plates of1 mm thickness and an Sn plate of 1 mm thickness sandwiched between theCu plates. The thin plate was cut to a size of 140 mm×1000 mm and thenplated on its one side with a Ti layer of about 10 μm thickness to formthe composite body 22d. The composite body 22d was then wound severaltimes around an Nb base metal rod 23a of 10 mm diameter and 1000 mmlength in the form of round bar. The resultant was inserted into anoxygen-free copper pipe 35 of 18 mm inner diameter and 19 mm outerdiameter, and drawn to form a hexagonal composite single core wire of4.2 mm across flats.

A Cu billet similar to that used in Example 6 was stuffed with 1225pieces of the composite single core wire thus formed, air in the billetwas evacuated, and a cover was welded to the billet by electron beamwelding. This billet was subjected to cold hydrostatic extrusion andthen drawn to form a composite wire. The resultant composite wire wasinserted into a Ta pipe serving as a barrier material against Sndiffusion, and further inserted into a Cu pipe for stabilization toachieve secondary composition, followed by being drawn to have a finalwire diameter. The composite wire of the final diameter was twisted toyield a superconducting wire precursor of 0.3 mm diameter. The sectionalstructure of the thus obtained superconducting wire precursor was asshown in FIG. 17.

The thus obtained superconducting wire precursor was subjected to apreheating and then to a heat treatment at 600° to 800° C. for 50 to 200hours to form a superconducting wire having Nb₃ Sn superconductorproduced in the Nb base metal filament portion. The sectional structureof the thus obtained superconducting wire was similar to that shown inFIG. 13.

The superconducting wire thus obtained was measured for Jc and d_(eff)in liquid helium. The measurement revealed that the Jc value of thissupercoducting wire was 910 A/mm² in a magnetic field of B=12T and thed_(eff) value was 3 μm. When the superconducting wire thus obtained iscompared with an Nb₃ Sn superconducting wire containing Ti and obtainedby the prior art internal diffusion method by totally estimating the twoin terms of Jc/d_(eff), this superconducting wire of the presentinvention is found to have been improved 7.5 times the prior artsuperconducting wire.

EXAMPLE 12

A superconducting wire may be required to be improved in Jccharacteristic rather than in d_(eff) characteristic. To meet thisrequirement, such an arrangement is offered in this Example thatfilaments are disposed more tightly than in the foregoing Examples. Thisarrangement makes it possible to improve Jc characteristic though mutualcoupling of the Nb₃ Sn filaments occurs to some extent after the heattreatment thereby to increase d_(eff) characteristic.

FIG. 22 is an explanatory view showing a structure of a composite singlecore wire prior to being reduced in section area for use inmanufacturing a superconducting wire according to the second aspect ofthe present invention. The composite single core wire comprises Cu-Sncomposite body 22a, Nb base metal rod 23d containing 1% by weight of Ta,and Cu base metal pipe 36 containing 3% by weight of Ti.

First, in the same manner as in Example 8, a thin plate of 1.5 mmthickness was formed by rolling to integrate a stack of two Cu plates of1 mm thickness and an Sn plate of 1.5 mm thickness sandwiched betweenthe Cu plates. The thin plate was cut to a size of 150 mm×1000 mm andthen wound several times around an Nb base metal round rod 23d of 12.7mm diameter and 1000 mm length which contained 1% by weight of Ta. Theresultant was inserted into the Cu pipe 36 of 19 mm inner diameter and20 mm outer diameter containing 3% by weight of Ti, followed by beingdrawn to form a hexagonal composite single core wire of 4.2 mm acrossflats.

A Cu billet similar to that used in Example 6 was stuffed with 1225pieces of the single core wire thus formed, air in the billet wasevacuated, and a cover was welded to the billet by electron beamwelding. This billet was subjected to cold hydrostatic extrusion andthen drawn to form a composite wire. The resultant composite wire wasinserted into a Ta pipe serving as a barrier material against Sndiffusion, and further inserted into a Cu pipe for stabilization toachieve secondary composition, followed by being drawn to have a finaldiameter. The composite wire of the final diameter was twisted to yielda superconducting wire precursor of 0.3 mm wire diameter. The sectionalstructure of the thus obtained superconducting wire precursor was asshown in FIG. 11. The superconducting wire precursor was subjected to apreheating and then to a heat treatment at 600° to 800° C. for 50 to 200hours to form a superconducting wire having Nb₃ Sn superconductor in theNb base metal filament portion. The sectional structure of the thusobtained superconducting wire was similar to that shown in FIG. 13.

The superconducting wire thus obtained was measured for Jc and d_(eff)in liquid helium. The measurement revealed that the Jc value of thissuperconducting wire was 1250 A/mm² in a magnetic field of B=12T and then value was 56. When the superconducting wire thus obtained is comparedwith an Nb₃ Sn superconducting wire containing Ti and obtained by theprior art internal diffusion method, this superconducting wire of thepresent invention is found to have been improved in Jc characteristic byabout 40% and in n value about twice.

As apparent from the results shown in Examples 1 to 4, it is observedthat the superconducting wires prepared by the improved internaldiffusion method according to the present invention are improved aboutfour to about seven times in superconducting characteristics estimatedin terms of Jc/d_(eff) value as compared with a conventionalsuperconducting wire prepared by the prior art internal diffusionmethod.

When the superconducting wires obtained by the dispersed Sn diffusionmethod in Examples 5 to 12 are totally estimated in terms of Jc/d_(eff)value, they are found to have been improved in superconductingcharacteristics about five to about seven times the superconducting wireobtained by the prior art internal diffusion method. In particular, thesuperconducting wires prepared in Examples 8 to 12 by a method whereinthe Cu-Sn composite body was wound several times around the Nb round barhave remarkably improved superconducting characteristics such as Jcvalue, d_(eff) value and n value, and further have an excellentprocessability. This is conceivably because winding the composite bodyseveral times around the Nb bar improved the dispersion of Sn andavoided the formation of non-uniform distribution of Nb₃ Sn produced infilament, leading to improved superconducting characteristics.

Although the superconducting wires prepared in the foregoing Examples 5to 12 are those having a stabilizing layer of Cu and a diffusion barrierlayer of Ta, it should be understood that use of Al of high purity forthe stabilizing layer and Nb or V for the diffusion barrier layer arealso effective, and that the provision of the stabilizing layer anddiffusion barrier layer may be omitted.

Further, although a multiplicity of single core wires were integratedinto a secondary composite to produce a single wire in Examples 1 to 12,a multiplicity of the thus obtained secondary composite wires may befurther stuffed in a Cu tube to give a composite and then reduced insection area so as to accommodate heavy current. That is to say, theprecursor wire having a structure as shown in FIGS. 1, 9, 12 and thelike may be used instead of the single core wire, and a precursor can beprepared using it in a similar manner, whereby a superconducting wirehaving a heavy current capacity can be obtained.

In addition to the ingredients used in the Examples, other ingredientscan be used, for example, by replacing Nb with V and Sn with Ga, as setforth in the specification to obtain substantially the same results.

In the Nb₃ Sn superconducting wire according to the first aspect of thepresent invention, as has been described, the spacing between adjacentNb base metal filaments is enlarged to such an extent that mutualcontact of Nb₃ Sn filaments to be produced during the heat treatmentwill not occur in the region where the boundary of ε-phase bronze layerappears during the preheating treatment for diffusing the Sn base metalmaterial. Hence, mutual coupling of Nb₃ Sn filaments in thesuperconducting wire due to mutual contact thereof can be avoided, andfurther the effective filament diameter of the superconducting wire canbe reduced with a decrease in Jc value suppressed to minimum. Thisresults in a substantial reduction in hysteresis loss produced uponconduction of pulse current and leads to a superconducting coil ofimproved stability.

In the Nb₃ Sn superconducting wire precursor according to the secondaspect of the present invention, a composite material composed of a basemetal material X such as Sn or Ga and Cu is dispersedly disposed aroundindividual filaments of a base metal material Z such as Nb or V, wherebythe spacing between adjacent filaments can be increased by about 30% ascompared with that according to the prior art internal diffusion method.Hence, the probability of Nb₃ Sn filaments or the like coming intomutual contact or coupling in the superconducting wire produced by theheat treatment of the precursor can be remarkably reduced whilesubstantially reducing the value of effective filament diameter. Thisresults in a substantial reduction in hysteresis loss produced uponconduction of pulse current and leads to a superconducting coil ofimproved stability. Further, since the base metal X such as Sn need notbe diffused so far, superconducting compound such as Nb₃ Sn is uniformlyformed in each filament thereby improving the Jc value and n value. Inaddition, the time period of the preheating for the diffusion of Sn orthe like is shortened thereby contributing to a cost reduction.

What is claimed is:
 1. A method for manufacturing an Nb-Sn compoundsuperconducting wire comprising the steps of:(a) forming a compositebody comprising a columnar Cu base metal material, and a plurality oflayers of Nb base metal filaments embedded in said Cu base metalmaterial and concentrically arranged around a center portion thereof,wherein said Nb base metal filaments are arranged separately from eachother and the spacing between any adjacent Nb base metal filamentsexisting in a boundary region of an ε-phase bronze layer produced whenpreheated at a temperature of from 300° to 600° C. is larger than thatbetween any adjacent Nb filaments existing in other portions of said Cubase metal material; (b) forming a through-hole in said center portionof said composite body, and inserting an Sn base metal rod into saidthrough-hole; (c) drawing the resultant composite body to form asuperconducting wire precursor; and (d) heat-treating said precursor. 2.The method of claim 1, wherein the step (a) comprises the steps of:(a1)forming thick-wall single core wires each comprising an Nb base metalfilament covered with a thick wall of a Cu base metal material, andthin-wall single core wires each comprising an Nb base metal filamentcovered with a thin wall of a Cu base metal material; and (a2) insertinginto a Cu container a Cu base metal material rod at the center of saidCu container, said thick-wall single core wires in said boundary regionof ε-phase bronze layer, and said thin-wall single core wires in boththe inside and outside or in only the outside of the annular arrangementof said thick-wall single core wires in said boundary region.
 3. Themethod of claim 1, wherein the step (a) comprises the steps of:(a3)forming a plurality of apertures in Cu disks such that the spacingbetween any adjacent ones of the apertures located in a regioncorresponding to said boundary region of ε-phase bronze layer is largerthan that between any adjacent ones of the apertures inside and outsidesaid region corresponding to boundary region of ε-phase bronze layer;(a4) inserting into a Cu container said Cu disks in a stacked fashion;and (a5) inserting an Nb base metal rod into each of said plurality ofapertures.
 4. The method of claim 1, wherein the spacing between anyadjacent ones of the Nb base metal filaments in said boundary region ofε-phase bronze layer is 0.45 or more times the diameter of any one ofthe Nb base metal filaments.
 5. The method of claim 1, wherein theregion in which the spacing between any adjacent Nb base metal filamentsis larger than that in the other regions is a region defined betweencircles having radiuses of 0.7 time and 1.4 times the distance from thecenter of said precursor to said boundary of ε-phase bronze layerproduced when said precursor is preheated at 415° C.
 6. The method ofclaim 1, wherein each of said Nb base metal filaments contains 0.01 to5% by weight of at least one element selected from the group consistingof Ti, Ta, Hf, Mo, Zr and V.
 7. The method of claim 1, wherein said Cubase metal material contains 0.01 to 5% by weight of at least oneelement selected from the group consisting of Ti, In, Ge, Si and Mn. 8.The method of claim 1, wherein said Sn base metal rod contains 0.01 to10% by weight of at least one element selected from the group consistingof Ti, In, Ga, Ge, Si and Mn.
 9. A method for manufacturing asuperconducting wire comprising the steps of:(A) forming a composite rodin which a multiplicity of rods of a second base metal material Z areeach embedded in a composite body composed of a Cu base metal materialand a first base metal material X, said Cu base metal material and saidfirst base metal material X forming an alloy on heating, wherein saidcomposite body is arranged around each of said rods of a second basedmetal material Z so as to be interposed between neighboring rods; (B)drawing the composite rod to form a superconducting wire precursor; and(C) heat-treating the superconducting wire precursor.
 10. The method ofclaim 9, wherein said composite rod is formed by:(A1) stuffing a Cucontainer with said composite body composed of said Cu base metalmaterial and said first base metal material X capable of forming analloy with said Cu base metal material; and (A2) forming apertures inthe composite body lengthwise and filling said second base metalmaterial Z into each of the apertures.
 11. The method of claim 9,wherein said composite rod is formed by:(A3) forming composite singlecore wires in each of which said second base metal material Z is coveredwith said composite body composed of said Cu base metal material andsaid first base metal material capable of forming an alloy with said Cubase metal material; and (A4) stuffing a Cu container with the compositesingle core wires tightly.
 12. The method of claim 9, wherein saidcomposite body is one formed by rolling a stacked plate of said Cu basemetal material and said first base metal material X to integrate thesematerials, or one prepared by plating said first base metal material Xon at least one side of said Cu base metal material.
 13. The method ofclaim 11, wherein said composite single core wires are each obtained bycentrally drilling said Cu base metal material to form an aperture whichis in turn refilled with said second base metal material Z, and fillinga plurality of apertures provided in said Cu base metal material andexisting in the periphery of said second base metal material Z with saidfirst base metal material X.
 14. The method of claim 11, wherein saidcomposite single core wires are each obtained by rolling around saidsecond base metal material Z a stacked plate of said Cu base metalmaterial and said first base metal material X or a plate of said Cu basemetal material which is plated with said first base metal material X onat least one side thereof.
 15. The method of claims 9, wherein saidsecond base metal material Z contains at least one element selected fromthe group consisting of Ti, Ta, Hf, Mo, Zr and V.
 16. The method ofclaims 10, wherein said second base metal material Z contains at leastone element selected from the group consisting of Ti, Ta, Hf, Mo, Zr andV.
 17. The method of claims 11, wherein said second base metal materialZ contains at least one element selected from the group consisting ofTi, Ta, Hf, Mo, Zr and V.
 18. The method of claims 12, wherein saidsecond base metal material Z contains at least one element selected fromthe group consisting of Ti, Ta, Hf, Mo, Zr and V.
 19. The method ofclaims 13, wherein said second base metal material Z contains at leastone element selected from the group consisting of Ti, Ta, Hf, Mo, Zr andV.
 20. The method of claims 14, wherein said second base metal materialZ contains at least one element selected from the group consisting ofTi, Ta, Hf, Mo, Zr and V.
 21. The method of claims 9, wherein said firstbase metal material X contains at least one element selected from thegroup consisting of Ti, In, Ge, Si and Mn.
 22. The method of claims 10,wherein said first base metal material X contains at least one elementselected from the group consisting of Ti, In, Ge, Si and Mn.
 23. Themethod of claims 11, wherein said first base metal material X containsat least one element selected from the group consisting of Ti, In, Ge,Si and Mn.
 24. The method of claims 12, wherein said first base metalmaterial X contains at least one element selected from the groupconsisting of Ti, In, Ge, Si and Mn.
 25. The method of claims 13,wherein said first base metal material X contains at least one elementselected from the group consisting of Ti, In, Ge, Si and Mn.
 26. Themethod of claims 14, wherein said first base metal material X containsat least one element selected from the group consisting of Ti, In, Ge,Si and Mn.
 27. The method of claims 9, wherein said Cu base metalmaterial contains at least one element selected from the groupconsisting of Ti, In, Ge, Si and Mn.
 28. The method of claims 10,wherein said Cu base metal material contains at least one elementselected from the group consisting of Ti, In, Ge, Si and Mn.
 29. Themethod of claims 11, wherein said Cu base metal material contains atleast one element selected from the group consisting of Ti, In, Ge, Siand Mn.
 30. The method of claims 12, wherein said Cu base metal materialcontains at least one element selected from the group consisting of Ti,In, Ge, Si and Mn.
 31. The method of claims 13, wherein said Cu basemetal material contains at least one element selected from the groupconsisting of Ti, In, Ge, Si and Mn.
 32. The method of claims 14,wherein said Cu base metal material contains at least one elementselected from the group consisting of Ti, In, Ge, Si and Mn.